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Skeptic Tank Systems: SEPTIC TANK

Most local regulatory authorities require that a septic tank be located at least 50 ft from any potable water supply (assuming that the house, or a nearby house, obtains its drinking water from a well) and that it not be any closer than 5 ft to the house. It is also sensible to install the septic tank, and the absorption system as well, on the downwind side of the house, in case any future difficulties with the system should result in the exhalation of noxious odors from a broken sewer line or an overflowing septic tank. For the same reasons, the septic tank and the absorption system should be installed on land that slopes down and away from the house.

House Sewer

The connection between the house plumbing system and the inlet of the septic tank is made via a house sewer (Fig. 1), which must be made of cast iron for a distance within 5 ft of the house. Beyond this distance, the house sewer can be made either of cast iron (the preferred material), vitreous clay tile, plastic, or concrete. A cast-iron sewer line should be installed no matter how close the septic tank is to the house if there is a potable water supply located within 100 ft of the sewer line or if trees are growing near the sewer line. The great advantage of cast-iron piping under these circumstances is that it is the material least likely to crack, which would allow raw sewage to seep into the soil. Also, roots cannot easily work their way between the pipe joints and thus force breaks in the pipe.

The house sewer is either 6 in. (preferred) or 4 in. in diam. If a 6-in, line is installed, it must slope at the rate of at least 1 in. per 8 ft. If a 4-in, line is installed, it must slope at the rate of at least 1 in. per 4 ft. In northern parts of the United States where the soil is likely to freeze, the house sewer must also be located at least 1 ft underground to prevent its being damaged by soil heave.

At the same time, because the absorption system must be as close to the surface of the earth as possible, the house sewer cannot be installed too deeply underground. It is, therefore, usually installed just under the first floor of a house so that it can enter the soil as close to the surface of the earth as possible. If the house should have a basement in which a laundry room or sink is located, the high position of the line will create a problem since any wastewater draining from the clothes washer or sink must be able to drain into the house sewer. What is usually done under these circumstances is to install a sump pump that will lift the water to the sewer line.

There is no compelling reason why the septic tank must be located close to the house at all, apart from the cost of laying a sewer line between them, If the land should slope away from the house at the rate of at least 1 in. per 4 ft, the septic tank can be located several hundred feet away from the house, if necessary. All that is required with a long downhill run of pipe is that the final loft or so of pipe not slope more than about 1 in. per 4 ft to prevent having the sewage discharge into the tank at too high a velocity.

Sizing a Septic Tank

The size of a septic tank is determined basically by the number of bedrooms in a house, as shown in Table 1, for it has been discovered that there is a direct relationship between the number of bedrooms in a house and the amount of sewage produced. Most health codes require that the septic tank have a minimum capacity of 500 to 1000 gal, regardless of the number of bedrooms. Since the usual assumption regarding water usage is that each person in a dwelling will on the average use 50 gal of water per day, it may be seen how conservative these requirements are.

Table 1. Determination of Septic Tank Size by Number of Bedrooms and by Use of Garbage Disposals



Tank size, gal

No. of bedrooms

Without disposal units

With disposal units










For each additional bedroom, add:



Why, then, is such a large tank required? Because a large tank ensures that freshly discharged sewage will remain in the tank long enough—at least 24 hr—for it to be completely decomposed by the bacteria in the tank, Because a large tank provides a margin of safety when there is a very large but temporary flow of sewage into the tank, as during a large house party, a wedding reception, and so on. Because it has been the experience of most communities since World War II that the per capita consumption of water tends continually to increase. Appliances such as garbage disposal units, automatic dish washers, and automatic clothes washers have come on the market that have greatly increased the per capita water consumption. A great many homeowners also add additional bedrooms and bathrooms to their homes as their fortunes and families wax in size. Where once a household was satisfied with but a single bathroom, two or three are now demanded. And since it would be foolish to dig up and replace a septic tank every few years because of this gradual but constant increase in sewage output, most communities now require that the original installation be made large enough in the first place to accommodate these anticipated increases in sewage output.

Septic Tank Design

A septic tank must have certain characteristics if it is going to be able to handle adequately the sewage that enters it. Basically, the tank must provide the placid and airtight environment that is necessary if the sewage is to be decomposed completely. The flow of liquid through the tank must be slow and steady. The particles of sludge must be allowed to settle quietly to the bottom of the tank where they may rest undisturbed; at the same time, the particles of scum must be allowed to rise to the surface of the liquid.

This tranquil environment is achieved in part by having the sewage enter and leave the tank through inverts, or baffles, which are the T-shaped cast-iron fittings shown in Fig. 2. The inverts are integrally cast into the walls of a concrete tank, or, in a steel tank, they are welded to the end walls. The incoming sewage can thus enter the tank with a minimal amount of turbulence and close to the bacteria-rich layer of sludge at the bottom of the tank.

Most tanks are also rectangular in shape, which further helps to induce a calm, unruffled flow of liquid through the tank. Most local regulatory authorities require that the tank have a certain minimum rectangular shape. For example, a tank may be required to be at least 21/2 ft wide and 5 ft long, with the bottom of the inlet T at least 4 ft from the bottom of the tank. This latter requirement helps to prevent the incoming sewage from disturbing the layer of sludge lying on the bottom of the tank.

Both the inlet and outlet T’s usually extend about 12 in. above the level of the liquid. This extension allows the scum to build up into a very thick layer over a period of time without its either backing up into and clogging the house sewer or entering into and clogging the tiles in the absorption system. Having a thick layer of scum also has a positive function in that it keeps the air at the top of the tank away from the anaerobic bacteria in the liquid. The inverts also allow the gases that are generated by the decomposing sewage to find their way out of the tank past the layer of scum.

Large tanks are often divided by a baffle into two components, as also shown in Fig. 2, their purpose being to further tranquilize the flow of liquid through the tank. In the usual range of tank sizes required for household use, a compartmented tank is unnecessary. An integral baffle does, however, help to strengthen the construction of a tank regardless of its size. Because most of the sludge and scum will accumulate in the first compartment, this compartment is usually required to contain at least two-thirds the total liquid capacity of the tank.

Dosing or Siphon Chamber

During the normal operation of a septic tank, the effluent is discharged from a full tank at exactly the same rate at which raw sewage enters the tank. Every time a water closet is flushed or the dishes are washed after dinner, 5 gal or so of wastewater will enter the tank. As a result, 5 gal or so of effluent will be discharged into the absorption system.

Because of the weak, intermittent nature of this discharge, the soil at the head of the absorption system tends to receive most of the effluent. If the soil should be only semi-permeable, it will tend to become saturated with effluent. This soil will never dry out, and the bacteria that collect in it (because of the limited amount of air that can reach the bacteria) will in time cause the soil to become rank and foul-smelling.

Given these circumstances, it would be wise to install a dosing, or siphon chamber, as part of the original septic tank installation (see Fig. 6). A dosing chamber is a temporary storage tank in which the effluent can collect. At periodic intervals the dosing chamber w* empty automatically, the effluent being discharged with some force into the absorption system. As a result, all the soil in which the absorption system is buried will receive an equitable distribution of the effluent. No one area of the soil will have a chance of becoming overloaded with effluent, air will have a chance to get at and destroy all the bacteria, and the filtered and aerated liquid will have time to drain away completely before the next dose of effluent arrives.

The operation of the dosing chamber depends on an automatic siphon installed in the chamber (see Fig. 6). Several firms manufacture these devices, which are made of cast iron in patented designs. Figure 6 shows the basic principle. If we assume that effluent enters a dosing chamber from which the liquid has been freshly discharged, the liquid level will begin to rise in the chamber as wastewater is discharged from the house plumbing system. As it does, air is trapped within the straight section of pipe, above the U-shaped trap. As the liquid level continues to rise, the hydraulic pressure exerted on the air column by the liquid will increase until a point is reached at which the hydraulic pressure forces the air down and through the trap. The fluid in the dosing chamber will immediately rush through the automatic siphon and out into the absorption system.

This flow will continue until the liquid level in the dosing chamber falls below the bottom edge of the bell housing. The siphoning action is thereupon broken, the flow of liquid into the absorption system ceases, air is again trapped within the pipe, and the dosing chamber will begin to fill with effluent again. If the siphon should become blocked, then the effluent will flow through the bypass line shown in the illustration and into the absorption system.

Fig. 6. Dosing chamber as part of a septic tank.

Whether or not a dosing chamber is really necessary will depend on the type of soil and the type of dispersal system installed. If the soil is very permeable, a dosing chamber will be unnecessary for most households. But the less permeable the soil, the greater the necessity for a dosing chamber. In addition, as the capacity of the septic tank increases above 1000 gal, a dosing chamber becomes more and more of a necessity. If a sand filter is being installed, a dosing chamber becomes obligatory if the sand filter is to operate efficiently. When a seepage pit is installed, a dosing chamber is unnecessary. To sum up, though dosing chambers are not a necessity in most household installations, they are still very useful devices to have.


Only certain organic substances, chiefly animal foods and wastes consisting of proteins, carbohydrates, and fats decompose easily in a septic tank. A great many other substances that are often unthinkingly disposed of by flushing them down a sink or water closet are not. For this reason, any household that depends on a septic tank for getting rid of its sewage is limited in what it can get rid of via this route.

Cellulose materials, for example, chiefly paper towels and sanitary napkins, but also including such fibrous vegetables as cabbage, lettuce, celery, carrots, and the like, are difficult to decompose. They should never be flushed into the drainage system but collected as solid wastes and disposed of as such. If paper towels and/or sanitary napkins are habitually flushed down a water closet, there is an excellent chance that they will pass through the septic tank and become lodged somewhere in the absorption system, thereupon blocking it. To clear the blockage, it may become necessary to dig up the drain tiles.

It is customary, nowadays, in many communities to install a ¼-in mesh screen on the outlet invert of the septic tank to prevent the passage of solid matter into the absorption system. Although solid wastes are kept out of the absorption system, they will accumulate as part of the sludge deposit. Their presence in the septic tank will increase the rapidity with which the sludge deposit builds up, and the septic tank will also be more difficult to clean out.

In the ordinary household, the amount of grease and oil that enters the septic tank is too small to be of any consequence, although these substances are very difficult to decompose. Some households, however, are heavy users of cooking oil, or the members may be in the habit of dumping used deep-frying oil into the kitchen sink. Most of this oil and grease ends up as part of the layer of scum in the septic tank, but many small particles do not. These particles are carried by the effluent into the dispersal system, and, over a period of time, they may gradually clog the soil into which the effluent leaches, thus reducing the effectiveness of the absorption system. In such households, a grease trap should be installed in the drain line between the kitchen sink and the house sewer.

Other substances that should never be discharged into a septic tank system include acids, gasoline, caustic compounds such as lye, and disinfectants. All these substances will destroy the anaerobic bacteria, which of course makes the septic tank system inoperative. Sewage will simply pass through the tank unchanged until the bacteria have had a chance to increase in numbers again.

Nor should a septic tank system be used to drain rainwater. If a roof drainage or soil drainage system is connected to the inlet of a septic tank, the rate of flow through the tank during rainy weather will become so great that both the anaerobic bacteria and the sewage will simply be flushed out of the tank and into the dispersal system before decomposition can occur.

The same thing, but to a much lesser extent, happens when a garbage disposal unit and/or an automatic clothes washer and/ or automatic dishwasher drain into the septic tank. If any of these appliances is installed in a house, a larger-than-normal- sized tank must be installed in anticipation of larger-than- normal flows of water (see Table 1). Another reason for having a larger septic tank, especially when a garbage disposal unit is installed, is that the unit is used to grind and flush into the plumbing system solid food wastes such as bones and vegetable peelings that would otherwise be collected and disposed of as garbage. As already mentioned, a septic tank is not intended to decompose these substances.

Septic Tank Construction

The walls of a septic tank must be strong enough to sustain the pressure of its contents without cracking and be durable enough to last for a great many years—20 years is not an unreasonable minimum. Most septic tanks are made of rein forced concrete, the walls of the tanks being at least 5 in. thick in the smaller sizes and increasing to 8 or 9 in. in the larger sizes. The walls are heavily reinforced with steel rods to increase their strength. Concrete tanks may either be constructed in place or they may be purchased prefabricated, requiring only a large crane and a crew of men to lower them into holes especially dug for them. A concrete tank though practically indestructible, can be attacked by some of the gases produced during the decomposition of the sewage. For this reason, to protect the concrete, the inside of the tank above the liquid level should be painted with an asphalt compound.

Steel tanks are also widely used. They are cheaper than concrete tanks because they are easier to fabricate and trans port, but the fact that they are made of steel makes them vulnerable to corrosion. The tank must be thickly painted both inside and out to protect the metal against corrosion, but, even so, a steel tank will last on the average only 10 to 15 years.

Septic tanks are also made of such decay-resistant woods as redwood and cedar. Ready-made tanks of these woods are sold knocked-down and ready to assemble. They are much cheaper than either concrete or steel tanks and are said to have a lifetime of 20 years, but since the tanks have not the strength of either concrete or steel, and as the possibility always exists that a seam may open up between the planks because of soil pressures outside the tank or liquid pressures within, most communities forbid their use.

Septic tanks made of plastics are also coming into use. Plastics have the advantage that they are impervious to corrosion or decay of any kind. They are not particularly strong materials, however, and they must be protected against crushing loads. For example, once installed in place, a plastic tank must be filled with water before soil is back-filled around it. Otherwise, the weight of the soil may crush the tank.


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